1. Field
This invention pertains to optical measurement equipment, and more particularly for a model eye for verifying proper operation and performance of optical measurement equipment, and a method for verifying proper operation and performance of optical measurement equipment with a model eye.
2. Description
There are a number of optical measurement or analysis instruments which utilize one or more light spots generated from coherent light sources, such as lasers or superluminscent laser diodes (SLDs), to make optical measurements of the eye. Well-known examples of such instruments include wavefront aberrometers (e.g., Shack-Hartmann wavefront aberrometers) and corneal topographers.
An undesirable feature of these light sources is that the light pattern produced in the instrument is marred by speckle. Speckle is a spotty pattern with large light intensity variations. FIG. 1 illustrates an example of a speckle pattern. Speckle is caused when the layer from which the light is scattered is thinner than the coherence length of the light source. A typical SLD has a bandwidth of thirty nanometers, which corresponds to a coherence length of one hundred microns.
Speckle can cause problems with some optical measurement or analysis instruments. For example, there are two ways that speckle causes measurement errors in an instrument that employs a Shack-Hartmann wavefront sensor. One problem is that the mathematical algorithms called reconstructors that are employed by such instruments have fitting errors in data sets that contain dark regions of the speckle pattern. Another problem is “intensity coupling.” Intensity coupling may occur when a wavefront sensor is constructed such that the lenslet array is not located exactly one focal length from the pixel array. In that case, intensity variations cause shifts in the spot locations that are independent of the slope of the wavefront. These shifts cause errors in the calculated wavefront.
With the human eye, speckle is mitigated because the scattering occurs in a volume that has a thickness that is longer than the coherence length of the light source. The light penetrates into a layer of the eye and weak scatter occurs throughout the volume. As a result, when an SLD light source illuminates a human eye, the bright-to-dark ratio is typically about two to one.
Meanwhile, it is sometimes necessary to be able to verify correct operation and specified performance of an optical measurement instrument such as a wavefront aberrometer in an operational setting. In many instances, this is done by operating the measurement instrument to make a measurement of a model eye whose characteristics are known. In that case, typically the optical measurement instrument injects a probe beam into a front surface of the model eye. Light scatters from the back surface of the model eye similarly to the way it does with a human eye, and some of the scattered light travels back out of the front surface and into the optical measurement instrument.
However, when a typical model eye is measured, the speckle is more severe than is typically seen with a human eye. The problem is made even worse by the fact that the “cornea” of the model eye acts like a magnifying glass and makes the structure of the bright spots and dark regions appear large on the detector inside the optical measurement instrument. The typical speckle pattern for a model eye has a bright-to-dark ratio of twenty-to-one, which is about an order or magnitude greater than for a human eye. These large variations in light level make for inaccurate measurements, for the reasons explained above.
Therefore, it would be desirable to provide a model eye which produces a speckle pattern exhibiting a reduced bright-to-dark ratio.
In one aspect of the invention, a method comprises: providing a model eye comprising an optically transmissive structure having a front curved surface and an opposite rear planar surface, and a material structure adhered to the rear planar surface of the optically transmissive structure; directing a coherent light beam through the front curved surface of the optically transmissive structure to the opposite rear planar surface; receiving at a measurement instrument a portion of the light returned from the rear planar surface of the optically transmissive structure, wherein the material structure has a characteristic to cause the light received by the measurement instrument from the rear planar surface of the optically transmissive structure to have a speckle pattern with a bright-to-dark ratio of less than 2:1.
In another aspect of the invention, a model eye comprises: an optically transmissive structure having a front curved surface configured to receive a coherent light beam and an opposite rear planar surface for directing a portion of the coherent light beam back out the model eye through the front curved surface; and a material structure adhered to the rear planar surface of the optically transmissive structure and having a characteristic to cause a speckle pattern of the portion of the coherent light beam that is directed back out the front curved surface of the optically transmissive structure to have a bright-to-dark ratio of less than 2:1.
In yet another aspect of the invention, a model eye includes: an optically transmissive structure having a front curved surface and an opposite rear planar surface; and a fabric-reinforced polyethylene pressure-sensitive tape with a semi-flexible shell adhered to the rear planar surface of the optically transmissive structure by a pressure sensitive adhesive.